A conventional safety valve means for a battery is constructed as shown in FIG. 1.
In FIG. 1, a lid plate 2 has therein a valve hole 1 and a thin seal plate 3 overlaps the lid plate 2 for covering the valve hole 1. A terminal plate 4 is formed in a reverse dish shape and overlaps on the thin plate 3. A turned edge 5 at the periphery of the lid plate 2 is bent so that the peripheral edge of the thin plate 3 is held between the lid plate 2 and the terminal plate 4.
The terminal plate 4 has therein a gas removing hole 6 and a portion of the terminal plate 4 is downwardly turned to form a cutter means 7. The thin plate 3 and the terminal plate 4 constitute a valve chamber 8. The lid plate 2, the thin plate 3 and the terminal plate 4 constitute a closure cover 9.
The closure cover 9 is inserted, through insulating packing 12, into the opening of an outer case 11 incorporating an electrode assembly 10. The opening edge 13 of the outer case 11 is inwardly turned so that the closure cover 9 is mounted to the outer case 11 in an air- and liquid-tight manner.
In assembling the closure cover 9 of such conventional structure, i.e. when the peripheral turned edge 5 of the lid plate 2 is bent, there the thin plate 3 is upwardly bent at the center portion thereof as shown in FIG. 1, due to bending pressure. This may cause the thin plate 3 to come in contact with the cutter means 7, so that the thin plate 3 is broken. On the other hand, even if the thin plate 3 does not come in contact with the cutter means 7 during the assembly, the swelling of the thin plate 3 due to bending of the thin plate 3 may cause it to come in contact with the cutter means 7 and be broken before the gas pressure in the battery reaches the predetermined value.
Furthermore, since bending conditions of thin plates are not uniform in conventional batteries, the gas pressure to cause the thin plates to be broken is not constant and predictable.
Thus, such conventional safety valve means lacks reliability.